Frequency generator for electrical discharge machining

ABSTRACT

A frequency generator for use with electrical discharge machining devices to afford automatic control of the operational duty cycle, such control being effected by monitoring cutting conditions at the cutting area or gap, reducing the duty cycle whenever the gap voltage falls below a predetermined voltage which is substantially in excess of zero voltage to automatically reduce the predetermined normal duty cycle by an amount proportional to the amount by which said gap voltage falls below said predetermined voltage, the monitoring means being continuously operable to monitor the gap voltage and thereby maintain the duty cycle at a level reduced from the normal duty cycle by an amount proportional to the amount which the gap voltage is below the predetermined voltage and to adjust the duty cycle to a predetermined minimum duty cycle if the gap voltage is reduced to zero volts.

United States Patent [191 Malesh July 10, 1973 FREQUENCY GENERATOR FORELECTRICAL DISCHARGE MACHINING [7 5] Inventor: Allan B. Malesh,l-lomewood, lll.

[73] Assignee: Abet Industries Corp., Broadview,

ill.

[22] Filed: Sept. 3, 1971 [21] Appl. No.: 177,803

[52] US. Cl 219/69 C Primary Examiner-R. F. Staubly Attorney-Lee J.Gary, Charles F. Pigott, Jr. et al.

[57] ABSTRACT A frequency generator for use with electrical dischargemachining devices to afford automatic control of the operational dutycycle, such control being effected by monitoring cutting conditions atthe cutting area or gap, reducing the duty cycle whenever the gapvoltage falls below a predetermined voltage which is substantially inexcess of zero voltage to automatically reduce the predetermined normalduty cycle by an amount proportional to the amount by which said gapvoltage falls below said predetermined voltage, the monitoring meansbeing continuously operable to monitor the gap voltage and therebymaintain the duty cycle at a level reduced from the normal duty cycle byan amount proportional to the amount which the gap voltage is below thepredetermined voltage and to adjust the duty cycle to a predeterminedminimum duty cycle if the gap voltage is reduced to zero volts.

17 Claims, 1 Drawing Figure o i +2Ov P2 er/770 f 3 Q8 r22 1 PM I 515 Ei551 ii Pas 11194 i 1w;

R21 63 if P1 Patented July 10, 1973 how A: Q I Q Q MR SR I QR fi h AvloR MWR E6 wwm EA \3MNW o wwk \Skmo I FREQUENCY GENERATOR FOR ELECTRICALDISCHARGE MACHINING BRIEF SUMMARY OF THE INVENTION The present inventionrelates generally to electrical discharge machining, and moreparticularly to the provision of a frequency generator with automaticcontrol of its operational duty cycle for use with electrical dischargemachining devices, hereafter sometimes referred to as EDM machines.

Typical frequency generators for use with EDM machines provide aselection of frequencies in the range from 250 Hz to 200 KHz to pulsethe associated power supply circuitry on and off for use in an EDMmachining process. The wave form supplied must be of the square wavetype with fast rise and fall times.

In a conventional EDM machining operation there is a small gap betweenthe electrode or cutting tool and the workpiece, and depending upon themanufacturer of the equipment d. c. pulses of from 80 to 150 volts areapplied to the gap resulting in a certain number of sparks per secondacross the gap. In accordance with conventional practice, the space atthe gap between the electrode and the workpiece is controlled so as toadjust the voltage across the gap down to a range of 20 to 50 volts. Ifthe machine were 100 percent efficient, the number of sparks per secondwould coincide with the frequency of pulses supplied by the generator.However, efficiency rates of 70 or 80 percent are commonly found,resulting in a proportionately lower number of sparks per second at theoperational gap. The frequency of these pulses is determined by acontrol included within the frequency generator.

The operational frequency employed is dependent upon the nature of thecut to be made. Thus, for a roughing cut the lower frequencies areutilized, whereas for finer or finish cuts it is desirable to use higherfrequencies. The on time or width of each pulse is referred to herein asthe duty cycle. For example, a 90 percent duty cycle indicates that eachpulse is on for 90 percent of the time and off for percent of the time.

Sparks resulting from the application of such voltage pulses at the gapmove around so as to follow the short est distance between the electrodeand the workpiece, resulting in erosion of the workpiece in one area andthen a shift of the spark to a different area. The action of the sparkon the workpiece as it erodes the latter produces chips somewhatspherical in shape and referred to herein as spheres." As spheres areformed during an EDM machining operation they are flushed out of the gapby the liquid in which the electrode and workpiece are submerged.

As described above, the voltage at the gap is adjusted between and 50volts in order to obtain optimum cutting operation. If the spheres arenot flushed out of the gap, they tend to build up and cluster togetherattracting the spark from the electrode toward the shorter path whichthey provide to the workpiece. Such action will have the effect ofreducing the voltage across the gap, and the resultant concentration ofspark at one location can cause serious damage to the workpiece as wellas to the EDM power supply.

In preparing for an EDM machining operation, the operator will adjustthe frequency control in accordance with the type of material to bemachined and the type of cut to be made. In addition, the operatorselects a desired duty cycle, sometimes starting with an adjustment onthe low side and subsequently increasing the predetermined duty cycleafter it has been determined that the sphere buildup is not excessive.

EDM machines conventionally are equipped with a servo system whichmonitors the voltage at the gap and controls the spacing at the gap soas to maintain a predetermined gap voltage. As described above, therange of voltage is normally between 20 and 50 volts, and the powersupply servo amplifier is adjusted to provide the desired voltage withinsuch range. The servo system monitors the actual voltage and compares itwith the set reference voltage, and it then backs the electrode away ifthe gap voltage is too low or advances the electrode closer to theworkpiece to reduce the gap if the voltage is too high.

However, the foregoing known apparatus is not a satisfactory solution togap problems since particles or spheres will sometimes accumulate alongthe side of an electrode, particularly when making a deep cut. Undersuch circumstances, backing off the electrode will not necessarily freesuch spheres, with the result that continued sparking will occur at theaccumulated particles or spheres as the electrode is backing away fromthe workpiece, thereby causing damage to the latter. Moreover, frequentbacking away of the electrode increases required machining time andresults in additional problems relative to reestablishment of properconditions at the gap when the electrode is again advanced.

Certain known EDM machines incorporate a current cut-off device for thepurpose of protecting against gap problems. Such a device monitors thecurrent at the gap, and if the current rises too high it will shut offthe current for a period of time. Altemately, voltage at the gap may bemonitored and the current removed if the voltage falls below acceptablelimits. Such arrangements shut off the current completely when a problemdevelops at the gap, and then after a predetermined time the current isrestored. Obviously, if the problem at the gap still exists, the circuitbreaker or switching operation will be repeated to again remove currentfrom the work area. Such action will continue until the gap problem iseliminated. However, such current cutoff devices are subject to thedisadvantage that difficulty will often be encountered in reestablishingcutting conditions at the gap when the current is restored. In anothervariation of the foregoing known apparatus, power cutoff based uponmonitoring of gap conditions has been done in steps rather than as asingle operation. Thus, transistors have been employed for controllingthe supply of power to the gap, but reducing current in this mannereffectively changes the output on impedance to the power supply withattendant problems often resulting when normal conditions are restored.

In accordance with the present invention, cutting conditions aremonitored at the gap and the duty cycle is automatically adjusteddirectly proportional to gap conditions. Thus, by means of apotentiometer, on time may be adjusted between 10 and percent. When thegap voltage drops below 20 volts, the duty cycle on time begins todecrease from its predetermined setting. The greater the drop below 20volts, the greater the reduction of the duty cycle from its initialsetting, in contrast with conventional machines where the initial dutycycle once adjusted remains fixed at the selected value. At zero volts,the duty cycle on time is at a minimum of 5 percent on. Retention of a 5percent pulse on time is requird so that a pulse signal is still at thegap which can be monitored by the gap sensor to enable the circuit tocontinue to determine the gap condition. As the gap is cleared ofdebris, and voltage is restored to the 20 volt operating level or above,the duty cycle will be returned to its original setting. A built-in timelag in the change of the duty cycle in either direction eliminatesunstable conditions at the gap which would occur if the duty cycle werevaried too quickly.

When the duty cycle or on time of the pulses from the power supply arereduced, current is effectively limited to the gap, resulting in theproduction of smaller particles or spheres from erosion of theworkpiece. By reducing such particle size, effective operation of theflushing system to clear out the particles is permitted thus preventingfurther buildup of additional particles. Moreover, under suchconditions, sparks within the gap may also serve to break up a largerparticle into smaller spheres. Where spheres or particles haveaccumulated causing a problem at the gap, it is undersirable to continuewith full current, since the spheres themselves may function as anelectrode and thereby cause damage to the workpiece. As soon as theproblem has been eliminated and the gap voltage is restored to 20 voltsor more, the duty cycle is restored to its initial setting.

The present invention reduces required machining time and affordsprotection of the integrity of the workpiece and protection for thepower supply by preventing excessive current overload. In accordancewith the invention, by monitoring the voltage at the gap and takingcorrective measures, a smooth limitation of current is provided with abuilt-in time delay that discounts any situation where gap voltagemomentarily fluctuates up and down. Such time delay permits reading ofan average voltage and changes the duty cycle accordingly.

As is the case with conventional EDM machines, the duty cycle may be setinitially between and 90 percent. The amount of automatic reduction ofthe duty cycle depends upon how far the voltage drops below 20 volts.For example, if the duty cycle is set at 90 percent V and the voltageremains at 20 volts or higher, no change will occur in the 'duty cycle.However, if the voltage drops to 10 volts, i.e., one-half of the amountfrom 20 volts to zero volts, then the duty cycle will be reduced byone-half the amount from the set 90 percent duty cycle to the minimum 10percent duty cycle, i.e., a reduction to a percent duty cycle. Animportant advantage of the present invention is that it may be added toexisting EDM machines without extensive modification, and will result inimproved operation thereof.

It is therefore an object of the present invention to provide animproved power supply for an electrical discharge machining device whichfunctions to monitor the voltage at the gap between the EDM electrodeand the workpiece being machined and which reduces the duty cycle of theapplied pulses when'such gap voltage falls below a predetermined level.

A further object is to provide an improved EDM power supply as lastabove-mentioned where the amount by which the duty cycle is reduced isproportional to the amount by which the gap voltage falls below a givenpredetermined value.

The foregoing and other objects and advantages of the invention will beapparent from the following description of a preferred embodimentthereof, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWING The single FIGURE comprises a schematiccircuit diagram of a frequency generator constructed in accordance withthe present invention. I

Now, in order to acquaint those skilled in the art with the manner ofmaking and using my invention, I shall describe, in conjunction with theaccompanying drawing, a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawing, anunderstanding of the present invention may be had by reference to thefollowing description of its arrangement and operation.

A frequency generator is shown including a gap monitoring circuitcomprising transistors Q9 and Q10 both of which function as amplifiers,with their associated components including resistors R24, R23 and R9 anddiodes D1 and D6. Field effect transistor Q1 and unijunction transistorQ2, along with their associated components comprising resistors R1 to R3inclusive, potentiometer RE and capacitors Cl and CEl to CE6 form arelaxation oscillator with the value of capacitor CE being selectable topermit selection of several operating frequencies.

The output of the foregoing oscillator is coupled through capacitor C1to the base of transistor Q3. The transistor Q3 is associated withresistors R4 and R5, and potentiometers PM] to PM6 which are utilized toadjust the minimum duty cycle of the generator. The foregoing componentscomprise an emitter follower amplifier the output'of which is appliedthrough a coupling network comprising resistor R6 and capacitor C2 tothe base of transistor ()4 which with its associated resistors R7 and R8form another emitter follower amplifier.

The output of the foregoing stage is coupled through resistor R10 to adifferential amplifier comprising'transistors Q5 and Q6, resistors R11through R15 and potentiometers P2 and P3. The remaining circuitryincludes transistors Q7 and Q8, resistors R16 through R22 inclusive,capacitor C4 and diodes D2 through D5, which function as a squaringcircuit which creates a fast rise and fall time to the square wave whichis required for the output stages of the power supply.

Switch S1 including sections 81A and SIB are employed to select one ofthe potentiometers PR1 through PR6 which are utilized to determine themaximum duty cycle of the present pulse generator. Switch S1, sectionSIC, is utilized to select one of the capacitors CEl to CE6 inclusivewhich determine the basic operating frequency of the output of thegenerator of the present invention. Switch S1, sections 81D and SlE areutilized to select one of the potentiometers PMl through PM6 inclusiveto determine the minimum duty cycle of the generator. It will thus beunderstood that the above switch S1, sections 81A, 51B, SID and 81B, areemployed to determine particular minimum and base duty cycle values.Since such switches are connected to potentiometers, the particularvalues selected may be further adjusted by operation of thepotentiometers themselves. In normal usage, the potentiometers willinitially lection of such predetermined values can be made by theoperator through operation of the switch S1 and potentiometer P1.

The above-described circuitry functions in the following manner: Fieldeffect transistor Q1 acts as a constant current source chargingcapacitor CEl, or capacitors CB2 to CB6, with the rate of current flowbeing adjusted by potentiometer RE. Unijunction transistor Q2 functionsto produce a linear ramp or sawtooth wave output train of pulses thefrequency of which is determined by capacitor CEl. Such pulses arecoupled through capacitor C1 to the base of transistor Q3 which as notedpreviously functions as an emitter follower amplifier. At this samepoint one side of resistor R4 is connected with the other side of R4connected to potentiometer PMl, or PM2 to PM6, whose adjustmentdetermines the minimum duty cycle of the generator. Potentiometer PM1 isnormally adjusted to yield a 10 percent duty cycle at the output of thegenerator when the operators control potentiometer P1 is turnedcounterclockwise. The negative potential thus derived and applied to thebase of transistor Q3 permits shifting the peak amplitude of the outputpulses appearing at the emitter of transistor Q3 from 2 to l5 volts.These pulses are coupled through an RC network comprising C2 and R6 tothe base of transistor Q4. Also connected to the base of transistor Q4is a potential of from O to +5 volts coupled through resistor R7 whichis connected to the arm of potentiometer P1. Adjustment of potentiometerP1 determines the basic duty cycle of the generator, i.e., from 10 to 90percent on time. As potentiometer P1 is rotated, the voltage varies fromto volts causing the ramp signal at the base of Q4 to be shifted up ordown as P1 is operated back and forth.

The train of pulses is taken from the emitter of Q4 through resistor Rinto the differential amplifier, conducting the train of pulses to thebase of transistor Q5. The d.c. bias level for the base of transistor O5is adjustable by means of potentiometer P2 which is included in avoltage divider that also includes resistor R11 and R12. Through suchdivider the d.c. bias is adjusted to a level of -1 volt. The emittervoltage of Q5 and Q6, derived from volts through potentiometer P3 andresistor R13, is O.4 volts, so that without an input signal into thebase of transistor Q5, the same will be reverse biased thereby turningit off and permitting transistor O6 to conduct through forward bias froma voltage divider network including resistors R15, R16 and R19.

During the initial portion of the cycle the voltage level is below -0.4volts, the emitter voltage of Q5 and Q6. As the voltage rises above 0.4volts, transistor Q5 becomes forward biased causing it to conduct andforcing transistor Q6 to turn off. At the end of the cycle the signaldrops below 0.4 volts at which time Q5 turns off and transistor Q6 isagain rendered conductive. At the collector of transistor Q6 the voltagedrops when it conducts causing transistor Q7 to be forward biasedpermitting conduction. When transistor Q6 is turned off, its collectorvoltage rises causing transistor Q7 to be reverse biased thereby turningit off.

The rapid switching of the differential amplifier comprising transistorsQ5 and Q6 causes a square wave to form at the output of transistor Q6.It is this output that is applied to the base of transistor Q7. If theoperator has potentiometer P1 set for a 50 percent duty cycle, the rampsignal to the input of Q5 shifts from -2.5 volts to +2.5 volts for eachcyclev As may be noted, transistor Q7 will thus be caused to conduct 50percent of the time and be cut off 50 percent of the time, yielding a 50percent duty cycle on time at the output of the circuit, because the0.4v triggering point of Q5 is midway between 2.5v and +2.5 volts.

The portion of the circuitry described thus far comprises a frequencygenerator with a voltage controlled duty cycle which may be manuallyadjusted by potentiometer Pl as required. As previously described, thevoltage derived from potentiometer P1 shifts the ramp signal at the baseof transistor Q4 which in turn determines the duty cycle as transistorsQ5 and Q6 conduct alternately. Potentiometers PR1 to PR6 are connectedat one end to ground or zero volt potential, while their opposite endsare connected to the +20 volts bus through resistor R9.

The foregoing components comprise a votlage divider with approximately+l9 volts appearing across the potentiometers, since the value of eachpotentiometer is ten times that of resistor R9. The arm of the desiredpotentiometer PR is adjusted so that a +5 volt potential is available atone end of potentiometer P1. At the junction of resistor R9 and the PRpotentiometers, the collector of transistor Q9 is connected throughdiode D1. The emitter of transistor Q9 is connected to ground so thattransistor Q9 and diode D1 are in parallel with potentiometer PR. Acoupling resistor R23 connects the collector of transistor Q10 to thebase of transistor Q9 with the emitter of transistor Q10 connected tothe +20 volt bus.

The condition at the gap of an EDM machine is monitored by connectingthe voltage appearing across the gap through resistor R24 to the base oftransistor Q10. As long as the gap voltage remains above +20 volts,transistor Q10 will-remain reverse biased and will not conduct. When thegap voltage falls below +20 volts, transistor Q10 will become forwardbiased and begin conducting. Relatively high values are employed forresistors R23 and R24 so that conduction of transistors Q9 and Q10increases linearly as the gap voltage falls from +20 volts toward zerovolts. Because of such linear conductivity, transistor Q9 graduallyreduces to ground the voltage at the junction of potentiometer PR1,resistor R9 and diode D1, thereby alsocausing the voltage at the arm ofpotentiometer PR to drop to zero volt.

A predetermined delay in the drop of voltage across potentiometer P1occurs due to capacitor C3 which in its fully charged condition acts tomaintain a +5 volt potential at the potentiometer P1. Eventually suchstored voltage discharges through one-half of the potentiometer PR1,through diode D1 and transistor Q9 to ground. When capacitor C3 has avalue of 500 microfarads, a 3 rfisecond delay is obtained in the reduction to zero voltage. As soon as the gap voltage is restored to above+2O volts, transistors Q9 and Q10 will turn off and capacitor C3 willrecharge up to +5 volts, at which time the duty cycle will be back atits normal or desired setting. If the gap voltage drops, for example, to+l0 volts, the duty cycle will fall to midway between the desiredsetting and the minimum value, and it will remain at such level untilthe gap voltage changes in either direction and will then followaccordingly.

Because of the rapid switching of the differential amplifier comprisingtransistors Q5 and Q6, a square wave output is generated, which in turnis applied to the base of transistor Q7 from which an output is derived.The remaining circuitry including transistors Q7 and Q8 and theirassociated components take the square wave obtained at the collector oftransistor Q6 and produce a wave form that has faster rise and falltimes than that obtainable at the output of transistor Q6.

Assuming the input signal at the base of transistor Q is below the pointat which conduction starts, transistor Q6 will conduct causing thevoltage on its collector to drop to approximately +19 volts. Since thebase of transistor Q7 is tied at this point, and its emitter is at +20volts, transistor Q7 will be forward biased allowing conduction. As maybe observed, there are two current paths from transistor Q7, the firstbeing from +20 volts through transistor Q7 and resistors R17 and R18 tothe 20 volt bus. The second path is from +20 volts through transitor Q7,resistor R17, diodes D2 through D5 and resistor R21 to the -20 volt bus.

The emitter and base of transistor Q8 are connected across diode D2, andbecause of the above-described current flow through diode D2, transistorQ8 will be reverse biased and thus non-conductive. Under suchconditions, the output taken through resistor R22 will be volts. Suchpotential is derived at the junction of resistors R16 and R19 which incombination with resistor R15 form a voltage divider network. The baseof transistor Q6 is also tied to the foregoing voltage divider betweenresistors R16 and R15 so that transistor Q6 receives positive feedbackfrom the generator outp t- When transistor Q6 is conducting, the outputis positive, which makes the base of transistor Q6 positive, causing itto'be driven further into conduction. When the output is at a negativelevel, transistor Q6 will not conduct because its base will be morenegative than its associated emitter, and being tied to the outputthrough resistor R16, it is forced to be more negative to keep Q6 fromconducting.

When the signal at the base of transistor 05 rises above the 0.4Vtripping point mentioned earlier, transistor Q5 will begin to conductcausing transistor Q6 to turn off whereby the collector voltage of thelatter will be approximately volts. Since the emitter of transistor O7is tied directly to the +20 volt bus, transistor Q7 will no longer beforward biased and will turn off. The two current paths previouslymentioned from transistor Q7 to the -20 volt bus thus cease to flow. Thebase of transistor Q8 is at a -l7 volt level derived from a voltagedivider comprising resistor R20, diodes D3 through D5, and resistor R21,with the base of transistor Q8 connected between resistor R20 and diodeD3. The emitter of transistor O8 is connected to 20 volts throughresistor R18 and thus is forward biased permitting conduction oftransistor Q8.

A path therefore exists from the -20 volt bus through resistorRl8,'transistor Q8 and resistor R19. This path forms a voltage dividernetwork across the +20 and 20 volt supplies, with the output connectedbetween transistor Q8 and resistor R19. Such divider network has the endeffect of causing the output of the generator to drop to a -l 3 voltlevel without load,giving a bipolar output from the generator. Byemploying high speed switching transistors for Q7 and Q8, and notdriving them into saturation, fast rise and fall times are therebyachieved in the present circuitry.

I claim:

1. For use with an electrical discharge machining device, a frequencygenerator comprising:

a pulse source operated to produce a continuous train of pulses at apredetermined frequency;

output means connected to said electrical discharge machining device,operable to produce a continuous train of bipolar pulses for conductionto said electrical discharge machining device;

duty cycle control means connected between said pulse source and saidoutput means, initially adjustable manually to determine the duration ofeach of said pulses produced by said pulse source;

said output means operated in response to said pulses of adjustedduration;

monitoring means connected between said electrical discharge machiningdevice and said duty cycle control means, operated in response todetection of a decrease in potential at said electrical dischargemachining device, below a predetermined value to automatically adjustsaid duty cycle control means to vary the duration of said pulses fromsaid pulse source whereby said output means is further operated to varythe duration of said bipolar pulses conducted to said electricaldischarge machining device, from the manually determined duration; and

said monitoring means being operable in response to a reduction in gapvoltage at said electrical discharge machining device below apredetermined voltage substantially in excesss of zero voltage toautomatically reduce the duty cycle from a predetermined normal dutycycle by an amount proportional to the amount by which said gap voltagefalls below said predetermined voltage, said monitoring means beingcontinuously operable to monitor said gap voltage so as to continuouslymaintain said duty cycle at a level reduced from said normal duty cycleby an amount proportional to the amount which said gap voltage is belowsaid predetermined voltage and to adjust said duty cycle to apredetermined minimum duty cycle if said gap voltage is reduced to zerovolts.

2. A frequency generator as claimed in claim 1 wherein said train ofcontinuous pulses produced by said pulse source are of saw-tooth waveform.

3. A frequency generator as claimed in claim 1 wherein said pulse sourceincludes means for manually selecting the frequency of operation of saidpulse source.

4. A frequency generator as claimed in claim 1 wherein said duty cyclecontrol means include selection means manually operated to select theduration of said pulses.

5. A frequency generator as claimed in claim 4 wherein said selectionmeans include first adjustable means for determining the basic durationof said pulses.

6. A frequency generator as claimed in claim 4 wherein said selectionmeans include second adjustable means for determining the minimumduration of said pulses.

7. A frequency generator as claimed in claim 4 wherein said selectionmeans include third adjustable means for determining the maximumduration of said pulses.

8. A frequency generator as claimed in claim 4 wherein said selectionmeans include first adjustable means for determining the basic durationof said pulses, second adjustable means for determining the minimumduration of said pulses and third'adjustable means for determining themaximum duration of said pulses.

9. A frequency generator as claimed in claim 4 wherein said monitoringmeans include potential magnitude detecting means connected between saidelectrical' discharge machining device and said selection means, saiddetecting means being operated in response to detection of saidpotential magnitude at said device being different than a predeterminedvalue to automatically further adjust said selection means toredetermine the duration of said pulses.

10. A frequency generator as claimed in claim 1 wherein said outputmeans comprise a differential amplifier connected between said dutycycle control means and said electrical discharge machining device, saiddifferential amplifier being periodically operated in response to pulsesof adjusted duration from said duty cycle control means to conductbipolar pulses to said electrical discharge machining device.

11. A frequency generator as claimed in claim 10 wherein said outputmeans further include shaping means connected between said differentialamplifier means and said electrical discharge machining device, saidshaping circuit being operated to modify the waveform of said bipolarpulses conducted to said electrical discharge machining device.

12. For use with an electrical discharge machining device, a frequencygenerator comprising:

a pulse source operated to produce a continuous train of pulses at apredetermined frequency;

output means connected to said electrical discharge machining device,operable to produce a continuous train of bipolar pulses for conductionto said electrical discharge machining device; duty cycle control meansconnected between said pulse source and said output means, initiallyadjustable manually to determine the duration of each of said pulsesproduced by said pulse source, said duty cycle control means includingselection means manually operated to select the duration of said pulsesincluding adjustable means for determining the maximum duration of saidpulses; and

monitoring means connected between said electrical discharge machiningdevice and said duty cycle control means, operated in response todetection of a decrease in potential at said electrical dischargemachining device, below a predetermined value to automatically adjustsaid duty cycle control means to vary the duration of said pulses fromsaid pulse source whereby said output means is further operated to varythe duration of each of said bipolar pulses conducted to said electricaldischarge machining device from the manually determined duration, saidmonitoring means including potential magnitude detecting means connectedbetween said electrical discharge machining device and said adjustablemeans, said detecting means being operated in response to detection ofsaid potential magnitude at said device being less than a predeterminedvalue to automatically further adjust said adjustable means toredetermine the maximum duration of said pulses.

13. For use with an electrical discharge machining device, a frequencygenerator comprising:

a pulse source operated to produce a' continuous train of pulses at apredetermined frequency;

output means connected to said electrical discharge machining device,operable to produce a continuous train of bipolar pulses for conductionto said electrical discharge machining device;

duty cycle control means connected between said pulse source and saidoutput means, initially adjustable manually to determine the duration ofeach of said pulses produced by said pulse source;

said output means operated in response to said pulses of adjustedduration and comprising a differential amplifier connected between saidduty cycle control means and said electrical discharge machining device,said differential amplifier being periodically operatedin response topulses of adjusted duration from said duty cycle control means toconduct bipolar pulses to said electrical discharge machining device;

monitoring means connected between said electrical discharge machiningdevice and said duty cycle control means, operated in response todetection of a decrease in potential at said electrical dischargemachining device below a predetermined value to automatically adjustsaid duty cycle control means to vary the duration of said pulses fromsaid pulse source whereby said output means is further operated to varythe duration of each of said bipolar, pulses conducted to saidelectrical discharge machining device from the manually determinedduration;

said output means further including shaping means connected between saiddifferential amplifier means and said electrical discharge machiningdevice, said shaping circuit being operated to modify the waveform ofsaid bipolar pulses conducted to said electrical discharge machiningdevice; and

said shaping means including feedback means connected to saiddifferential amplifier, whereby said differential amplifier operationaltime is reduced in response to the polarity of pulses conducted to saidelectrical discharge machining device.

14. For use with an electrical discharge machining device, a frequencygenerator comprising:

a pulse source operated to produce a continuous train of pulses at apredetermined frequency;

output means connected to said electrical discharge machining device,operable to produce a continuous train of bipolar pulses for conductionto said electrical discharge machining device;

duty cycle control means connected between said pulse source and saidoutput means, initially adjustable manually to determine the duration ofeach of said pulses produced by said pulse source;

said output means operated in response to said pulses of adjustedduration and comprising a differential amplifier connected between saidduty cycle control means and said electrical discharge machining device,said differential amplifier being periodically operated in response topulses of adjusted duration from said duty cycle control means toconduct bipolar pulses to said electrical discharge machining device;

shaping means connected between said differential amplifier means andsaid electrical discharge machining device, said shaping circuit beingoperated to modify the waveform of said bipolar pulses conducted tosaidelectrical discharge machining devicefand said shaping meanscomprising a first switch having an input connected to said differentialamplifier, and a second switch including an input connected to saidfirst switch and an output connected to said electrical dischargemachining device, said first switch being operated upon operation ofsaid differential amplifier in response to the presence of a pulse ofadjusted duration to operate said second switch to conduct pulses of afirst polarity to said electrical discharge machining device, and saidfirst switch being further operated upon further operation of saiddifferential amplifier in response to absence of pulses of adjustedduration to operate said second switch to connect pulses of a secondpolarity to said electrical discharge machining device.

15. A method of controlling an electrical discharge machining operationso as to prevent damage to the workpiece and to the- EDM power supplywhen a breakdown occurs at the gap between the electrode and theworkpiece comprising the steps of sensing when the gap voltage fallsbelow a predetermined voltage substantially in excess of zero volts,reducing the duty cycle from a predetermined normal duty cycle by anamount proportional to the amount by which the gap voltage falls belowsaid predetermined voltage, continuously monitoring said gap voltage soas to continuously maintain said duty cycle at a level reduced from saidnormal duty cycle by an amount proportional to the amount which said gapvoltage is below said predetermined voltage, and adjusting said dutycycle to a predetermined minimum duty cycle when said gap voltage isreduced to zero volts.

16. A'method as defined in claim 15 where said predetermined voltage isapproximately 20 volts.

17. A method as defined in claim 15 where a time delay is providedbetween a change in said gap voltage and a corresponding change in saidduty cycle.

1. For use with an electrical discharge machining device, a frequencygenerator comprising: a pulse source operated to produce a continuoustrain of pulses at a predetermined frequency; output means connected tosaid electrical discharge machining device, operable to produce acontinuous train of bipolar pulses for conduction to said electricaldischarge machining device; duty cycle control means connected betweensaid pulse source and said output means, initially adjustable manuallyto determine the duration of each of said pulses produced by said pulsesource; said output means operated in response to said pulses ofadjusted duration; monitoring means connected between said electricaldischarge machining device and said duty cycle control means, operatedin response to detection of a decrease in potential at said electricaldischarge machining device, below a predetermined value to automaticallyadjust said duty cycle control means to vary the duration of said pulsesfrom said pulse source whereby said output means is further operated tovary the duration of said bipolar pulses conducted to said electricaldischarge machining device, from the manually determined duration; andsaid monitoring means being operable in response to a reduction in gapvoltage at said electrical discharge machining device below apredetermined voltage substantially in excesss of zero voltage toautomatically reduce the duty cycle from a predetermined normAl dutycycle by an amount proportional to the amount by which said gap voltagefalls below said predetermined voltage, said monitoring means beingcontinuously operable to monitor said gap voltage so as to continuouslymaintain said duty cycle at a level reduced from said normal duty cycleby an amount proportional to the amount which said gap voltage is belowsaid predetermined voltage and to adjust said duty cycle to apredetermined minimum duty cycle if said gap voltage is reduced to zerovolts.
 2. A frequency generator as claimed in claim 1 wherein said trainof continuous pulses produced by said pulse source are of saw-tooth waveform.
 3. A frequency generator as claimed in claim 1 wherein said pulsesource includes means for manually selecting the frequency of operationof said pulse source.
 4. A frequency generator as claimed in claim 1wherein said duty cycle control means include selection means manuallyoperated to select the duration of said pulses.
 5. A frequency generatoras claimed in claim 4 wherein said selection means include firstadjustable means for determining the basic duration of said pulses.
 6. Afrequency generator as claimed in claim 4 wherein said selection meansinclude second adjustable means for determining the minimum duration ofsaid pulses.
 7. A frequency generator as claimed in claim 4 wherein saidselection means include third adjustable means for determining themaximum duration of said pulses.
 8. A frequency generator as claimed inclaim 4 wherein said selection means include first adjustable means fordetermining the basic duration of said pulses, second adjustable meansfor determining the minimum duration of said pulses and third adjustablemeans for determining the maximum duration of said pulses.
 9. Afrequency generator as claimed in claim 4 wherein said monitoring meansinclude potential magnitude detecting means connected between saidelectrical discharge machining device and said selection means, saiddetecting means being operated in response to detection of saidpotential magnitude at said device being different than a predeterminedvalue to automatically further adjust said selection means tore-determine the duration of said pulses.
 10. A frequency generator asclaimed in claim 1 wherein said output means comprise a differentialamplifier connected between said duty cycle control means and saidelectrical discharge machining device, said differential amplifier beingperiodically operated in response to pulses of adjusted duration fromsaid duty cycle control means to conduct bipolar pulses to saidelectrical discharge machining device.
 11. A frequency generator asclaimed in claim 10 wherein said output means further include shapingmeans connected between said differential amplifier means and saidelectrical discharge machining device, said shaping circuit beingoperated to modify the waveform of said bipolar pulses conducted to saidelectrical discharge machining device.
 12. For use with an electricaldischarge machining device, a frequency generator comprising: a pulsesource operated to produce a continuous train of pulses at apredetermined frequency; output means connected to said electricaldischarge machining device, operable to produce a continuous train ofbipolar pulses for conduction to said electrical discharge machiningdevice; duty cycle control means connected between said pulse source andsaid output means, initially adjustable manually to determine theduration of each of said pulses produced by said pulse source, said dutycycle control means including selection means manually operated toselect the duration of said pulses including adjustable means fordetermining the maximum duration of said pulses; and monitoring meansconnected between said electrical discharge machining device and saidduty cycle control means, operated in response to detection of adecrease in potential at said electrical discharge machining device,below a predetermined valUe to automatically adjust said duty cyclecontrol means to vary the duration of said pulses from said pulse sourcewhereby said output means is further operated to vary the duration ofeach of said bipolar pulses conducted to said electrical dischargemachining device from the manually determined duration, said monitoringmeans including potential magnitude detecting means connected betweensaid electrical discharge machining device and said adjustable means,said detecting means being operated in response to detection of saidpotential magnitude at said device being less than a predetermined valueto automatically further adjust said adjustable means to redetermine themaximum duration of said pulses.
 13. For use with an electricaldischarge machining device, a frequency generator comprising: a pulsesource operated to produce a continuous train of pulses at apredetermined frequency; output means connected to said electricaldischarge machining device, operable to produce a continuous train ofbipolar pulses for conduction to said electrical discharge machiningdevice; duty cycle control means connected between said pulse source andsaid output means, initially adjustable manually to determine theduration of each of said pulses produced by said pulse source; saidoutput means operated in response to said pulses of adjusted durationand comprising a differential amplifier connected between said dutycycle control means and said electrical discharge machining device, saiddifferential amplifier being periodically operated in response to pulsesof adjusted duration from said duty cycle control means to conductbipolar pulses to said electrical discharge machining device; monitoringmeans connected between said electrical discharge machining device andsaid duty cycle control means, operated in response to detection of adecrease in potential at said electrical discharge machining devicebelow a predetermined value to automatically adjust said duty cyclecontrol means to vary the duration of said pulses from said pulse sourcewhereby said output means is further operated to vary the duration ofeach of said bipolar pulses conducted to said electrical dischargemachining device from the manually determined duration; said outputmeans further including shaping means connected between saiddifferential amplifier means and said electrical discharge machiningdevice, said shaping circuit being operated to modify the waveform ofsaid bipolar pulses conducted to said electrical discharge machiningdevice; and said shaping means including feedback means connected tosaid differential amplifier, whereby said differential amplifieroperational time is reduced in response to the polarity of pulsesconducted to said electrical discharge machining device.
 14. For usewith an electrical discharge machining device, a frequency generatorcomprising: a pulse source operated to produce a continuous train ofpulses at a predetermined frequency; output means connected to saidelectrical discharge machining device, operable to produce a continuoustrain of bipolar pulses for conduction to said electrical dischargemachining device; duty cycle control means connected between said pulsesource and said output means, initially adjustable manually to determinethe duration of each of said pulses produced by said pulse source; saidoutput means operated in response to said pulses of adjusted durationand comprising a differential amplifier connected between said dutycycle control means and said electrical discharge machining device, saiddifferential amplifier being periodically operated in response to pulsesof adjusted duration from said duty cycle control means to conductbipolar pulses to said electrical discharge machining device; shapingmeans connected between said differential amplifier means and saidelectrical discharge machining device, said shaping circuit beingoperated to modify the waveform of said bipolar pulses conducTed to saidelectrical discharge machining device; and said shaping means comprisinga first switch having an input connected to said differential amplifier,and a second switch including an input connected to said first switchand an output connected to said electrical discharge machining device,said first switch being operated upon operation of said differentialamplifier in response to the presence of a pulse of adjusted duration tooperate said second switch to conduct pulses of a first polarity to saidelectrical discharge machining device, and said first switch beingfurther operated upon further operation of said differential amplifierin response to absence of pulses of adjusted duration to operate saidsecond switch to connect pulses of a second polarity to said electricaldischarge machining device.
 15. A method of controlling an electricaldischarge machining operation so as to prevent damage to the workpieceand to the EDM power supply when a breakdown occurs at the gap betweenthe electrode and the workpiece comprising the steps of sensing when thegap voltage falls below a predetermined voltage substantially in excessof zero volts, reducing the duty cycle from a predetermined normal dutycycle by an amount proportional to the amount by which the gap voltagefalls below said predetermined voltage, continuously monitoring said gapvoltage so as to continuously maintain said duty cycle at a levelreduced from said normal duty cycle by an amount proportional to theamount which said gap voltage is below said predetermined voltage, andadjusting said duty cycle to a predetermined minimum duty cycle whensaid gap voltage is reduced to zero volts.
 16. A method as defined inclaim 15 where said predetermined voltage is approximately 20 volts. 17.A method as defined in claim 15 where a time delay is provided between achange in said gap voltage and a corresponding change in said dutycycle.